<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <div id="app"></div>
    <script src="react.js"></script>
    <script src="react-dom.js"></script>
    <script src="browser.min.js"></script>
    <script type="text/babel">


    //clock组件
    class Clock extends React.Component{
        constructor(props) {
            super(props);
            this.state = {
                date:new Date()
            }
        }
        componentDidMount() {
            this.timerID = setInterval(
                () => this.tick() , 
                1000
            )
        }
        componentWillUnmount() {
            clearInterval(this.timerID);
        }

        tick(){
            this.setState({
                date:new Date()
            });
        }
        render(){
            return (
                <div>
            <h1>Hello world</h1>
            <h2>
            It is {this.state.date.toLocaleTimeString()}.
            </h2>
            </div>
            )
        }
    }
    //调用render方法挂载组件。
    ReactDOM.render(
            <Clock />,
            document.getElementById("app")
        )
    </script>
</body>
</html>